If You Don’t Understand Quantum Physics, Try This!


Quantum physics has a mystique of being complicated
and hard to understand, in fact Richard Feynmann who won the Nobel prize for his work on quantum
electrodynamics said: “If you think you understand quantum physics, you don’t understand
quantum physics”. Which is kind of disheartening for us because if he didn’t understand it,
what chance do the rest of us have? Fortunately this quote is a little misleading.
We do in fact understand quantum physics really well, in fact it is arguably the most successful
scientific theory out there, and has let us invent technologies like computers, digital
cameras, LED screens, lasers and nuclear power plants. And you know, you don’t really want
to build a nuclear power plant if you don’t really understand how it works. So quantum physics is the part of physics
that describes the smallest things in our Universe: molecules, atoms, subatomic particles
thing like that. Things down there don’t quite work the same way that we are used to
up here. This is fascinating because you and everything around you is made from quantum
physics, and so this is really how the whole universe is actually working. I’ve drawn these protons, neutrons and electrons
as particles, but in quantum mechanics we really describe everything as waves. By the
way I’m using quantum physics and quantum mechanics interchangeably, they are the same
thing. So instead of an electron looking like this, it should look something like this.
This is called a wave-function. But this wave-function isn’t a real physical
wave like wave on water or a sounds wave. A quantum wave is an abstract mathematical
description. To get the real world properties like position or momentum of an electron we
have to do mathematical operations on this wave-function, so for the position we take
the amplitude and square it, which for this wave would look something like this. This
gives us a thing called a probability distribution which tells us that you are more likely to
find the electron here than here, and when we actually measure where the electron is,
an electron particle pops up somewhere within this area. So with quantum physics we don’t know anything
with infinite detail, we can only predict probabilities that things will happen, and
it looks like this is a fundamental feature of the Universe which was quite a departure
from the clockwork, deterministic universe in classical physics, the kind of thing Newton
derived. This wave-function model predicts what subatomic
particles will do incredibly well, but weirdly we’ve got no idea if this wave-function is
literally real or not. No one has ever seen a quantum wave because whenever we measure
an electron all we ever see is a point like electron particle. So there is like this hidden
quantum realm where the waves exist, and then the world we can see, which is where all the
waves have turned into particles. And the barrier between these is a measurement. We
say a measurement ‘collapses’ the wave function, but we don’t actually have any
physics to describe how the wave collapses. This is a gap in our knowledge that we have
dubbed the measurement problem, and this is one of the things that Feynmann was referring
to with his quote. Another confusing thing is how exactly to
picture an electron. It seems to be a wave until you measure it, and then it is a particle,
so what actually is it? This is known as particle-wave duality, and here is an example of it in action:
the famous double slit experiment. Imagine spraying a paintball gun at a wall
with two openings in it, you’d expect to see two columns of paint go through and hit
the wall behind. But if you shrink this all down to the size of electrons you see something
quite different. You can fire one electron at a time at the slits and they appear on
the back wall, but as they build up over time you get a whole pattern of stripes, instead
of just two bands, this pattern of stripes is called an interference pattern, something
you only see with waves. The idea is that it is the electron-wave that goes through
both slits at the same time, and then the waves from each slit overlap with each other,
and where the waves add together you have a high probability of the electron popping
up at the wall, but where the waves cancel out the probability is very low. So actually
on the back wall the highest probability of finding the electron is in the middle of the
slits, and then it goes down and up again, and down and up again and this is the interference
pattern. So when you fire one electron after another they follow this probability distribution
and this interference pattern starts building up, and that’s exactly what we see in experiments.
So this shows that electrons behave like waves in this experiment. A question is what actually happens to this
spread-out electron-wave when you do a measurement? It seems like it goes from this spread out
wave to this localised particle, but like I said, there’s nothing in quantum mechanics
that tells us how the wave-function collapses. And this is not only true for electrons, but
for everything in the Universe, so this double slit experiment has huge consequences for
our model of the Universe, and it was very surprising the first time it was done. Physicists
are still grappling with this question today and have come up with many interpretations
of quantum mechanics to try an explain these results, and explain how reality actually
works. Okay lets go back to the wave-function. Now
we can use this picture to explain other features of quantum physics that you may have heard
about. So this is just one possible wave-function
for an electron, but there are many others. Like this one for instance. This says that
the electron has a probability of being over here, and a probability of being over here,
and very little probability of being in the middle. This is perfectly allowable in quantum
physics and this is where the phrase ‘things can be in two places at once’ comes from.
This is known as superposition, which comes from the fact that this wave can be made by
adding, or superimposing these two waves. The word superposition just means the adding
together of waves and we already saw this in the double slit experiment, and is not
really a very special phenomenon. You can even see superposition by dropping two pebbles
into a pond where the ripples overlap. Now for entanglement. Let’s say two electron-waves
meet. Their waves interfere with each other and become mixed up. This means that mathematically
we now have one wave-function that describes everything about both electrons and they are
inextricably linked, even if they move far away from each other. A measurement on one
of the particles, like measuring if it is spin up or down is now correlated with a measurement
on the other, even if they move billions of miles away. Einstein was very uncomfortable
with this idea because if you measure one of the particles here you instantaneously
know what the other will be even if it is billions of miles away, and that’s got a sort
of whiff of faster than light communication, which is not allowed by the theory of relativity.
But it turns out you can’t actually use this to communicate information, because the
measurements give you random results, but the fact that they are correlated means that
somehow there is a link that stretches over that distance. This is called non-locality. Quantum tunnelling. Quantum tunnelling is
where particles have a probability of moving through barriers, essentially allowing things
like electrons to pass through walls. When a wave-function meets a barrier it decays
exponentially in the barrier, but if the barrier is narrow enough the wave-function will exist
on the other side meaning there is a probability of the particle being found there when a measurement
is made. In fact the only reason you are alive is because
of quantum tunnelling in the Sun which make the Sun shine. Protons normally repel each
other, but they have a small probability of quantum tunnelling into each other which is
what turns hydrogen into helium and releases fusion energy. All life on Earth exists because
of energy from the Sun, except for life around hydrothermal vents. Now on to the Heisenberg Uncertainty principle.
I said that the beginning that this wave-function contains all of the information like position
and momentum of the electron, we just have to do some maths on it. The position is given
by the amplitude, or height of the wave, and the momentum is given by the wavelength of
the wave. But for this specific wave the position gives
us a probability distribution, so we don’t know exactly where the electron is. Also there
is an uncertainty in the momentum because this wave is made of many different wavelengths. But we can reduce that uncertainty, let’s
have a wave that only has one wavelength, so a sine wave. Now we know the momentum exactly
because the wavelength has a single value, but look at the position. There is an equal
probability of the electron being found anywhere in the universe. Okay let’s do the opposite
let’s make a wave that has only got one position. Now we know exactly where the electron
is, but what is the wavelength of the wave? Now the wavelength is very uncertain. Basically
only a sine wave gives you a precise momentum, and any wave that isn’t a perfect sine wave,
you have to build out of multiple different sine waves, and each of those multiple different
sine waves has got a different wavelength, and hence you have a range of possible different
values of momentum for the particle. This is Heisenberg’s Uncertainty principle,
you can only know certain things precisely, but not everything. Either you have got a
definite value of momentum, and don’t know anything about position, or you know the position
very well, but don’t know anything about the momentum, or you are in some intermediate
state. And this isn’t a limit of our measuring apparatus, this is a fundamental property
of the Universe! And finally, where does the name ‘quantum’
come from. Well a quanta is a packet of something like a chunk of something, and one of the
first quantum effects people saw were atomic spectra which is where atoms give off light
with specific discrete energies. It works like this. Imagine a string that is tied at
both ends, like a guitar string. If you pluck it, only certain waves can exist because the
ends are tied down, in this situation we say that the wavelengths are quantised to certain
values. The same thing happens if you ties the ends
of the string together because the waves have to match up, they can only vibrate in certain
restricted ways. And this is what is happening to an electron in an atom. The electron-wave
is constrained by the atom and quantised to certain wavelengths, short wavelength have
high energy and long wavelengths have a lower energy. This is why the light emitted by an
atom looks like a barcode because each bar of light corresponds to an electron jumping
from a wave with a high energy to one with a lower energy, and at the same time emitting
a quantised photon of light when it does this. So the light from an atom is quantised to
discrete packets of energy. Okay so that’s all the basics of quantum
physics, here are some technical notes which aren’t essential to know, but pause the
screen now if you are interested in a little more mathematical detail. So to round up. In quantum physics objects
are described with wave-functions, but when we measure them, what we see are particles,
so this leads to particle-wave duality, and also the measurement problem. And the consequence
of these wave-functions are the quantum phenomena of superposition, entanglement, quantum tunnelling,
the Heisenberg uncertainty principle and energy quantisation. So if you understand these things
you have got a good basic understanding of quantum physics. Despite its reputation I think quantum mechanics
isn’t too difficult for most people to get the basics of what is going on. In the past
I have relied upon analogies to try an explain it, but here I have just described what is
actually going on which I think might be more helpful. But if you have more questions I’ll
be on the comments below so ask away. For me the weird thing about quantum physics
is that on the one hand it is incredibly accurate and predictive but also it has got giant holes
in it like the measurement problem which we just don’t understand. So we can wonder,
will we ever actually understand quantum physics, or is it just too abstract for our human brains
to comprehend. Well I hope this video has helped you understand a little more about
how quantum physics works. And thanks to the sponsor of this video brillaint.org,
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